Well, after you explore the library AND catch Jacques trying to sneak into the tower, the following day, you can talk to Lisa about what you found in the library, but you can only move forward if you admit to everything
After that, when you try to talk to her again, you'll have nothing new to say to her. But if you try a third time, she'll ask "Ever find out what Hotchkiss lost?" Now to get her to say that, you have to talk to Hotchkiss and explore her room as well around the time you find Jacques.
Anyway, when you talk to Lisa and say "Yeah well she still won't tell me what's missing. Seems she's suspicious of everybody," then Lisa will get mad and refuse to talk to you anymore..
But after you go outside and find the key to the tower gate, you can check your voicemail, listen to it, and the third message will be one from Lisa. From there, you can talk to her about Dexter supposedly taking the medallion out to the shed. And that's that.
I honestly don't remember the game that well, since my walkthrough for the game was done eight years ago. Do people want to see me make a separate video, for this one phone message that I didn't trigger in my walkthrough?